1. Field of the Invention
The present invention relates generally to electronic circuits, and more particularly to the formation of electrical feed-throughs in a hermetic package such that conventional off-the-shelf (COTS) semiconductor components may be used in place of expensive, labor-intensive and less reliable chip-and-wire constructions.
2. Background
Certain military standards, such as MIL-STD-883 and MIL-PRF-38534, the contents of which are incorporated herein by reference, describe the requirements for high reliability hermetically sealed electronics modules which can withstand extraordinary conditions and variations of temperature, G-forces, humidity, etc., without significant instances of failure and consistent operability. The tests are rigorous, and include, inter alia: tests measuring reliability over an extended period of time at maximum operating conditions; measuring the impact of storage in an unloaded electrical condition at an elevated temperature; measuring the mechanical strength of electrical connections; measuring resistance to solvents; etc.
Because these electronics may be exposed to extreme environments, for example high and low temperatures, humidity, and dynamic forces involved in military, industrial and aerospace applications, there is an ongoing need for improvements in reliability. There is further the continuing need for improving the efficiency and economy of manufacturing these modules.
Until now, in order to provide circuits exhibiting extraordinary reliability in such extreme environments, it has been necessary to use hermetic packages based on a ceramic circuit board bearing electronic components in the form of special uncoated or "raw" silicon wafers or chips, to which leads are wire bonded. This manufacturing method is known as "chip-and-wire," and a typical unit can be seen in FIG. 1. Leads have been wire bonded to special feed-throughs or vias which provide a path for power and signals through the hermetic container wall. The circuit board substrate is then encased in a metal or ceramic cover and hermetically sealed, usually in dry nitrogen, to protect the electronics from the deleterious effects of moisture and oxygen. Problems exist in matching the coefficient of thermal expansion of the ceramic to an encasing metal and in stabilizing the ceramic/metal bonds without deleteriously affecting the electronics. It should further be noted that the manufacture of hermetic packages typically employs brazing and welding to ensure the sealing of internal-external feed throughs. The heat required for brazing can damage or destroy conventional circuit boards, soldered connections and circuits.
Protection from extreme ambient conditions is particularly important in marine or aerospace shipboard navigation and weapons guidance systems, where electronic malfunction can mean the failure to meet mission objectives, increased risk to military personnel and civilians, or catastrophic loss of equipment or human life.
Because of the extreme conditions and reliability required by military standards, it has not been possible to utilize COTS components in such modules. COTS electronics packages typically comprise a silicon wafer or chip potted or encased in a plastic composition to seal the wafer to some extent from the environment, with electrical leads extending through the plastic. When it is attempted to utilize such components in extreme conditions without sealing them hermetically, the electronics typically fail, e.g., because oxygen, moisture or other harmful materials diffuse through the plastic, and/or the plastic simply fails under the conditions of use. The potting material used in conventional COTS chips also undesirably "outgasses" volatiles as it ages. It is also difficult to use COTS components inside a hermetic package because it is particularly difficult to provide vias through the substrate, which is typically ceramic, which are aligned with the leads extending from the COTS package.
The advantages to using COTS components would reside in both cost and handling during manufacture. The chip and wire method requires the use of unpotted or raw silicon, which is far more expensive than potted chips because raw silicon must be specially handled by chip manufacturers. The commercial demand for potted chips is far greater than for raw wafers, therefore most manufacturers do not sell raw wafers off the shelf. Rather, when raw wafers are sold, they are specially removed from the manufacturing processes before the plastic packaging step. The wafers are removed from the process under stringent conditions, and maintained in stringent conditions until the final packaging. Thus the use of raw wafers adds significantly to the cost of manufacturing prior art packages. Firstly, the special handling in the factory renders raw wafers more costly. Further, because of their fragility, raw silicon wafers are far more expensive to handle without damaging the chip. All of these aspects contribute to the cost of the final product employing raw wafers.
A recurring problem in the prior art has been how to provide and extract electronic power and signals into and out of the hermetic package without violating the hermetic integrity of the sealed package. The structure which permits electronic exchange into and out of the package is usually called a "feed through" or "via". These terms are used interchangably in this specification.
One type of prior art feed through used in hermetic electronics systems employs a copper sphere placed in a hole drilled in the ceramic substrate. By sandwiching the sphere between two pieces of copper on either side of the substrate and applying pressure, the sphere is compressed into filling the hole in a sealing manner. In a so-called chip-and-wire package, raw silicon chips are then mounted to a substrate, and individual connections wired to these squashed-sphere vias, which may be machine-drilled to provide a soldering/wire bonding locus, to form the circuit. As many as 80 or more such feed throughs may be required for a particular sealed electronic module. This method is labor-intensive, low-yield, and therefore very expensive. In high-power applications, the cost per feed through may approach $10, with the resulting cost of the complete electronics module costing, for example, in the range of $600 to $1600 per unit.
Several other arrangements providing vias in an electronics package are disclosed in the prior art. For example, U.S. Pat. No. 5,640,051 discloses a package on a carrier which has vias terminating in contact electrodes. The via holes accept electrodes on a primary substrate such as to allow contact with electrodes. The primary substrate also comprises vias which terminate on the surface with a pad upon which the electrodes are disposed. This method would not be suited for use with hermetic packages because the vias and connections are for use with conventionally-soldered non-hermetic packages. The substrates are typically ordinary PC board, and are not designed for the extreme environments required for high reliability hermetically sealed packages.
U.S. Pat. No. 5,633,531 discloses the composition of glass beads used for hermetic package feed throughs. This conventional method reflects the current state-of-the-art for hermetic packages, which requires each feed-through terminal to be wire bonded to a chip mounted on the substrate, or to other parts of the circuit. Disadvantageously, the use of COTS components is not possible. Moreover, the state of the art does not allow for the use of pick-and-place robotics for assembly of these hermetic packages, such robots being efficiently used in conventional circuit manufacture.
U.S. Pat. No. 5,600,181 discloses a hermetic package having a sealed connector with leads connecting pads to the board. The leads are sealed in the ceramic. This method, while providing a feed-through sealed into the package, still mandates the use of the chip-and-wire assembly method. As mentioned previously, that method is extremely labor-intensive, requiring manual soldering of the leads to raw chips. Again, neither COTS components nor pick-and-place robots may be used in that method.
In conclusion, none of the prior art discloses a hermetic package which allows low-cost, high-reliability manufacture of a hermetically sealed package conforming to military specifications.